6th
International Science, Social Sciences, Engineering and Energy Conference
17-19 December, 2014, Prajaktra Design Hotel, Udon Thani, Thailand
I-SEEC 2014
http//iseec2014.udru.ac.th
Investigation on Metaradiator based on Metasurface
Sarawuth Chaimoola,e1, Tanan Hongnarab,e2, Prayoot Akkaraekthalinb,e3
a
Electrical Engineering and Electronics Technology, Faculty of Technology, Udon Thani Rajabhat University, Thailand
Electrical and Computer Engineering, Faculty of Engineering, King Mongkut's University of Technology North Bangkok, Thailand
b
e1
jaounarak@gmail.com, e2tanan.hongnara@hotmail.com, e3prayoot@kmutnb.ac.th
Abstract
Three unusual electromagnetics properties of metasurface namely negative permittivity (ENG), negative
permeability (MNG), and near-zero refraction index (NZI) have been investigated. These properties are
obtained by coupling effect of the combination between fractal fishnet structure and closed ring resonator
on different sides of unit cell. To better understand the characteristics of metasurface, the metasurface as
a superstrate/cover placed atop a conventional dipole at a small distance, which called metaradiator, has
been studied and demonstrated. Moreover, in order to understand and explain clearly the unusual
behaviors of metasurface, the robust retrieval method and the generalized sheet transition conditions have
been applied to extract effective material properties. Numerical and simulated results show three different
radiation patterns of metaradiator are strongly affected from metasurface within the entire band.
Keywords: metamaterial, metasurface, metaradiator, ENG, MNG, NZI
1. Introduction
Nowadays, the design and engineering of artificial composite structures named as Metamaterial have
sparked exponentially increased rapidly. Many phenomena and applications of metamaterials are related
to perfect imaging [1], invisible cloak [2], electromagnetic (EM) wave perfect absorbers [3], enhanced
efficiency of antenna [4]-[5], zero-ordered antenna [6], microwave filter [7] and artificial magnetic
permeability [8]. As the typical building blocks of metamaterials, the split ring resonator (SRR) and the
wire rod have been studied [9]. Their local resonances are supported by both metallic structures endow
them with role of electric and magnetic meta-atoms in constructing metamaterials of the wire rod and
SRR, respectively [10]. Besides, they have desired bulk EM behavior, therefore, metasurface or metafilm
has been suggested and extended from three-dimension (3D) to a two-dimension (2D) pattern at a planar
surface [11]. The metasurface is a 2D version of the bulky metamaterial structure which takes less
physical space and offers a less lossy structure for possible artificial control of electromagnetic waves
through it. As a new area of research, the cutting edge technology may lift up the designing and
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realization of metasurface towards novel functionalities and improved performance inclusion which may
facilitate obtaining increased bandwidths and reduced losses. These properties make metasurface able be
applied to microwave circuits and antennas.
Due to their intrinsic structures, the conventional wire rod and SRR cannot give the rise to a significant
ratio of the operating wavelength over their sizes. Moreover, it is well known that significantly
subwavelength electromagnetic resonators have been employed in antennas and microwave circuit
applications. In order to realize the high-performance metasurfaces, these meta-atoms or unit cells should
be made as subwavelength as possible [12]. Over a few years, fractal techniques have been widely used in
the design of frequency selective surface and metamaterials. Due to their self-similar property, the fractal
based metamaterials offer multiband operations. Also, since their space filling property, they can be used
to miniaturize the dimension of unit cell.
In this paper, we have investigated experimentally and numerically the electromagnetic characteristics
of the combination between Minkowski fractal fishnet structure and closed ring resonator as metasurface
in the microwave frequency range. The fractal scheme employed in this research is from the viewpoint of
a much electrically smaller unit cell of metasurface in virtue of the space-filling properly. Combining two
types of resonators formed the metasurface, it has ENG, MNG and NZI characteristics. In addition, we
also interpret the bulk permittivity and permeability to 2D effective electric and magnetic surface
susceptibilities. In order to better understand metasurface behaviors by putting a simple dipole antenna
closed to metasurface, we find that the antenna’s radiation patterns have different peaks of main beams
independently-controlled frequencies.
2. Characterizations of Metasurface
During the recent years, several methods have been proposed to extract the effective material
properties of metamaterial slab [12]-[13]. Some researchers have concentrated their work on easy and
robust retrieval method for effective constitutive parameters (permittivity, permeability and refractive
index) to characterize the metamaterial and metasurface. These techniques are generally based on the
inversion of the reflection and transmission parameters of a normally incident wave on the slab. After
obtaining the wave impedance (z) and effective refractive index (n), the scalar values of effective
permittivity (εeff) and effective permeability (µeff) are then calculated as µeff= n*z and εeff= n/z. However,
electromagnetic behavior of most metamaterial designs is anisotropic or bi-anisotropic, which require a
tensor for characterizing it. The recent technique developed by Szabo et. al. is based on Kramers-Kronig
relations and thus complied with the principle of causality [14]. While the conventional methods to model
3D-metamaterials is with effective medium theory [12]-[14], the characterization of metasurfaces was
suggested to use other ways [11], [16]-[19] due to its planar surface nature. The reason is that no unique
values of effective material properties can be obtained as they depend on the value of effective thickness.
One of the promising techniques for metasurface is the application of the generalized sheet transition
conditions (GSTCs) [17], where the averaged boundary conditions are applied for the specular interaction
of electromagnetic waves with a surface of electrically small scatterers. The electric and magnetic surface
susceptibilities, which associate the EM field response to the surface polarization densities of the
scatterers at the metasurface [18]. In this paper, we used both methods including robust method [14] and
GSTCs) [17] to compare and explain the characteristics of the proposed metasurface. In this particular
design, the metasurface is illuminated by a normally incident plane electromagnetic wave with Epolarization in the x direction as shown in Fig. 1. The proposed unit cell consists of two resonator types of
resonators namely closed ring resonator and fractal fishnet structure on two sides of square area of 18×18
mm2. On the top side, the second-iteration order of Minkowski geometry is applied as depicted in Fig. 1.
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The Minkowski fractal fishnet of the second-iteration order is utilized by taking a trade-off between
miniaturization and fabrication. On another side, a square closed ring resonator with the patch is used as
shown in Fig. 1. The advantage of this unit cell is supporting two linear orthogonal polarizations and also
the capability of independent control of permeability and permittivity of the bands that unusual
characteristics. Computations were performed with a commercial full-wave software (CST Microwave
Studio).
By studying the commonly used effective parameter extraction techniques, the effective constitutive
parameters, permeability (µeff) and permittivity (εeff) are extracted; the graphical presentation is given in
Figure 2(a). It is observed the effective εeff response that it is form Drude model [10] with εeff = 1-(p2/),
where p is the electric plasma frequency with εeff equal zero value. Also, due to high Q of permittivity,
the obtained characteristic rapidly changes through plasma frequency of 1.24 GHz with extreme high
slope. Meanwhile, the magnetic plasma frequency is 1.27 GHz from effective constitutive permeability.
This characteristic response is well known that Lorentz dispersive model [10]. It means that each side of
metasurface contains a periodic structure producing one of the resonant plasma frequencies. From above
results, it can be classified that the epsilon negative (ENG) behavior is the real part of εeff has negative
values while µeff is positive. The ENG frequency range is below 1.24 GHz. At the center of the refractive
index response, it has near zero value of the refractive index (NZI) that is perceived at frequency around
1.25 GHz. Vice versa, the frequency range above 1.27 GHz is claimed with negative value (MNG) of µeff
whereas εeff has positive values.
In another point of view, the electric and magnetic susceptibility parameters are extracted by using set
of equations [17] as shown in Fig. 2(b) and 2(c), where effective electric and magnetic surface
susceptibilities are symbolical as ES and MS, respectively. The parameters ES and MS are the dyadic
surface electric and magnetic susceptibilities that related the electric and magnetic polarizability densities
of the scatterers per unit area. It is clearly seen that ES and MS are virtually the same as the effective εeff
and µeff (Fig. 2(a)) with different positive and negative scales, respectively. It is seen that a strong electric
resonance occurs at 1.24 GHz compared with the magnetic response. The real part of MS becomes
positive above the plasma frequency. A positive magnetic surface susceptibility MS is obtained above
1.27 GHz for this perfect electric conductor scatterer. This implies a correct polarity for MS at which
diamagnetic materials was defined in [20].
3. Results and Discussion
To investigate and better understand the unusual behaviors of the proposed metasurface, we placed a
simple half-wavelength dipole as primary source closed to the metasurface. The metasurface consists of
25 unit cells in a 5×5 arrangement. The cross-sectional view and detailed view of the proposed antenna
with its optimized parameters are presented in Figure 3(a). For this obtained coordinate, the dipole has
omnidirectional pattern around y-axis. Actually, to guarantee the bandwidth of the final antenna, both the
metasurface and dipole should operate in a wideband with fully overlapped frequencies. However, in
order to ensure that the primary source has only the first mode with omnidirectional pattern. Therefore,
three dipoles with different resonant frequencies at 1.15 GHz, 1.29 GHz, and 1.40 GHz have been
designed. |S11| and radiation patterns are shown in Fig. 3(b). It can be seen that all three resonant
frequencies have omnidirectional patterns with gain of 2.0 dBi. Then, place the metasurface closed to
dipole where the closed ring resonators are faced to the dipole. The simulated normalized radiation
patterns at 1.15 GHz, 1.29 GHz, and 1.40 GHz are plotted in Fig. 4. The three distinct modes have
different radiation pattern characteristics as the location of peaks in the 3-D radiation patterns change. As
expected, the radiation patterns of each frequency are dramatically affected by the presence closely
spaced metasurface in its near-field region. It is evident that the proposed metasurface can work at 1.29
GHz as a transparent surface (reflectionless), frequency ranges below 1.29 GHz as reflector and above
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1.29 GHz as a director-like providing a phenomenon of anomalous reflection. It can be explained that this
director-like property makes it possible to use an external source to drive a surface current on the
resonator structure, so as to interfere with the current induced by the primary incident wave, leading to
changes in the far field radiation pattern.
It is concluded that metasurface can reflect the incident wave in anomalous way or control the
propagation of the incident wave. Generally speaking, the antenna can switch its radiation without any
switch. For most of the metasurface’s in the literature, unfortunately, it is not possible to control the bands
independently or arbitrarily. For instance, for the metasurface’s employing permeability near zero (MNZ)
was used in desired band [4].
4. Conclusion
To conclude, we have succeeded through both theory and experiment that different kinds of unusual
behaviors of metasurface can serve different beam directions as subwavelength reflector, transparent
surface and director-like surface. As a specific demonstration, a dipole with metasurface is able to control
its radiation pattern with different maximum beam directions depending on the unusual behaviors of
metasurface. As results, the proposed metasurface will open a new way to control the radiation pattern,
promising great potentials in modern wireless communication systems especially cognitive radios.
Moreover, with geometrical scalability, this concept can be further applied to millimeter-wave or optical
regimes.
Acknowledgements
This work has been supported by the Thailand Research Fund (TRF) through the TRF Senior
Research Scholar Program Grant No. RTA5780010).
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Fig. 1 Configuration and dimensions of the proposed metasurface. Metasurface is printed on FR-4 substrate with r = 4.2 and thickness of 0.8 mm.
(a)
(b)
(c)
Fig. 2 (a) Effective constitutive parameters using robust retrieval method and Generalized sheet transition conditions (GSTCs) (b) effective electric
surface susceptibility (ES) and (c) magnetic surface susceptibility (MS).
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(a)
(b)
Fig. 3 (a) Configuration of the dipole with metasurface and (b) |S11| and radiation patterns of the dipoles alone with different frequencies.
(a)
(b)
Fig. 4 Simulated 3–D radiation patterns (a) 1.15 GHz with ENG, (b) 1.29 GHz with NZI and (c) 1.40 GHz with MNG.
(a)
(b)
Fig. 5 Measured radiation patterns (a) 1.15 GHz with ENG, (b) 1.29 GHz with NZI and (c) 1.40 GHz with MNG.
(c)
(c)